Jump to content

Sign in to follow this  
Guest

VNAV SPEED/VNAV PATH/FMC SPEED question

Recommended Posts

Guest

Good morning all:Often on climb-out and descent, I have noticed that the Auto Pilot will direct the airplane or flight director (I fly these parts of the flight by hand with the AT engaged) to maintain speed with attitude adjustement (nose-up/nosed down), which frequently means I am missing the altitude targets at each waypoint - often by substantial amounts. Last night I hit my crusing alititude a good 70 miles before the ToC bug on the HSI and the plane would have kept going if I hadn't dropped out of VNAV. This also happens on descent, when I might hit my 12 miles out final approach alititude a good two waypoints early. It seems to me that this is likely an Auto Throttle issue - i.e. it is not programmed properly to direct the plane to each waypoint at the right speed and altitude. Can someone give me the skinny on how to get the AT and the AP working together on this - or can they? I often find myself dropping out of VNAV and manually entering rate of climb/descent and speed info directly into the AP via the MPC.The proceedure I am following is: Program the FMC, adjust any legs for altitude and speed and activate the route. Input weights, cruise altitude, cost factor, flap setting for take off, get V Speeds and bugs set etc. Set MCP speed for V2 + 20; set MCP altitude for cruising altitude. Turn on flight director and arm AT.Take off roll - hit N1 as the engines spool up to 40% - take hands off throttle handles. Rotate and and activate LNAV and VNAV buttons. Speed disappears in MCP window - needles start tracking the flight plan on the FD. Any help would be appreciated. There is clearly a fundamental concept that I am missing here - same thing happens on the Dreamfleet 737. Many thanks.

Share this post


Link to post
Share on other sites

This was once posted by Eric. Read it carefully:VNAV "ANOMOLIES"Here is a list of things that differ between our VNAV and the one found in the aircraft.(1) Our VNAV seems to work better than the real aircaft . Especially when it comes to a VNAV descent. The most common comment heard in the cockpit during a VNAV descent is "what the hell is it doing now!".(2) When a climb speed/altitude restriction is entered for a waypoint, the FMC incorrectly carries over the speed restriction to the subsequent waypoints. However, when the waypoint is crossed, VNAV properly resets the speeds to their proper target values. For example, if a speed restriction of 230 knots is placed on a waypoint prior to T/C, 230 will be listed for all subsequent waypoints. However, VNAV will properly restore the ECON speeds (or SEL speed if entered) after the waypoint has been crossed.(3) If a speed restriction ONLY has been added to a waypoint in the LEGS page during the climb, VNAV will ignore this speed restriction. I believe it is not possible in the real plane (Pegasus) to enter a speed restriction alone (without an altitude restriction) on the LEGS page. It is for sure not possible in a standard FMC equiped aircraft.(4) Entering a new CRZ ALT in the VNAV pages prior to dialing in the new altitude in the AFDS MCP causes VNAV to be removed and ALT HOLD engaged. The real aircraft would not dump VNAV in this case and would remain in VNAV PTH at the FMC CRZ ALT. The removal of VNAV mode can be avoided by resetting the MCP altitude first and then updating the CRZ ALT in the FMC. In this case, VNAV remains engaged and properly vertically navigates the plane.(5) VNAV descent to the first restricted waypoint is not a true "idle" descent. It is based on a 3:1 glidepath and some power is used to maintain the predicted path. The VNAV path is maintained through using a pre-selected target V/S. The IDLE annuncation should also be replaced by THR HOLD mode. FYI: The real aircraft uses an idle power descent in THR HOLD mode that is similar to FL CH mode.(6) After reaching the first restricted waypoint in a VNAV descent, the IDLE annunciation on the EADI should be replaced by SPD or N1 depending on the situation. This is because VNAV no longer bases further descents on idle power. Other than the annunciation problem, our VNAV works 100% the same as the real aircraft for VNAV descent after the first restricted descent waypoint.(7) Speed restrictions on the LEGS page for a waypoint are not complied with in a VNAV descent until after crossing the restricted waypoint. To work around this limitation, you could use speed intervention on the MCP to manually adjust the speed prior to crossing a restricted waypoint. Or you can create an along track waypoint prior to the restricted waypoint and place the speed restriction there.Michael J.

Share this post


Link to post
Share on other sites

Hi Michael. Just a couple of notes/corrections:3. Correct, it is NOT possible in the real plane to have a speed restriction without an altitude. A trick that real pilots use to achieve this is during the descent is to set an altitude for the waypoint that is well below what the actual altitude will be so that the altitude part of the restriction will have no effect on the descent profile.4. Are you sure about this? There is a difference in VNAV behaviour between aircraft that have the "altitude intervention" feature and those that do not. Our aircraft does not have the "alt intervention" feature.7. This was fixed in the FS2002 patch. The plane should slow down now in time to make the descent speed restrictions.Regards,Wade

Share this post


Link to post
Share on other sites
Guest

I'm not sure I understood everything up there, except the "What the hell is it doing!" quote - been there, done that. So, would I be better off calculating a basic rate of climb that will get me to my TOC at roughly the right alitutude (or to key waypoints in the SID) and using the MCP inputs for climb-out? Is this what they do up front in the real airplane?Thanks.Colin

Share this post


Link to post
Share on other sites
Guest ilh

Reading carefully the buttons you are pushing, I am not certain you are pushing a CMD button on the MCP. If you are not, the autopilot is not controlling the aircraft, but instead just driving the flight director. In this case, you are still hand flying.If you are pushing CMD, then I don't know what is going on. I have never seen VNAV climb beyond the cruise altitude or the MCP altitude. I find T/C to be computed reasonably accurately, although I am not sure it considers wind in its computation.Lee Hetherington (KBOS)[link:www.sls.lcs.mit.edu/ilh/fs/DFMS734.zip]B734 Dreamfleet/Microsoft Merge

Share this post


Link to post
Share on other sites

You may be right Wade about (4). Entering the new CRZ altitude by itself will not trigger the HOLD mode. Only when the aircraft is actually prevented to go higher due to restriction on the MCP panel will HOLD mode be engaged. Last night it did actaully happen to me - I entered the new CRZ altitude (39000) but my MCP was reading 37000 and I forgot to change it. The aircraft was in climb but when I came back to my room it was in ALT HOLD at 37000.

Share this post


Link to post
Share on other sites

Going into Alt Hold when reaching the MCP altitude if the MCP alt is less than vnav crz alt (or more than in descent) is the correct behaviour for an aircraft that does not have the "altitude intervention" feature. This is the major difference between those that have it and those that don't. This is another of those airline options, it isn't specific to the plane. You must then change the MCP alt and re-engage vnav to continue the climb/descent. This is not the case for legs altitude restrictions, in that case the plane will level off and stay in VNAV.Regards,Wade

Share this post


Link to post
Share on other sites
Guest

Thank you for the very detailed reply. I appreciate you taking time to type that out. I am hand flying the plane up to cruise altitude (and down again) using the flight director in LNAV and VNAV modes. So, I don't press command until I'm at cruising altitude.I am wondering if the allowed deviation from the flight path is such that, if you get too far out the system gets "confused?" Just a thought.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

×
×
  • Create New...